The present invention relates to a frequency synthesis method for an AM-SSB transmitter-receiver having a phase locked loop (PLL) synthesizer and, more particularly, to a frequency synthesis method for an AM-SSB transmitter-receiver excellent in stability, spurious properties and economy using two kinds of frequencies including a frequency for producing an AM carrier frequency and the upper sideband of SSB, selected in common in the frequency control system of the PLL synthesizer before modulation, and for producing the lower sideband of SSB.
In the SSB system, an offset oscillator and a mixer are used for producing an offset frequency signal whose upper and lower sidebands are apart from each other; when carrier frequencies are not to be synthesized from the sum of the offset frequency and voltage-controlled oscillation frequency and the difference therebetween, it is necessary to install additional mixers and crystal oscillators with economical difficulties in consideration of the economical optimum operating conditions of the crystal oscillator, spurious properties and optimum frequency applicable to the variable frequency divider; and carrier frequencies for transmission and reception are produced from the reference frequency, offset frequency and voltage-controlled oscillation frequency, and therefore it is necessary to give as high frequencies as possible to the upper and lower sideband frequencies so as to prevent deterioration of the spurious properties.